1. Field of the Invention
The present invention relates to a test apparatus for testing the quality of an electronic device. More particularly, the present invention relates to a test apparatus for testing the quality of an electronic device wherein the internal clock of the electronic device has jitter.
2. Related Art
Recently, the trend towards a high speed electronic device such as a semiconductor device is considerable. For example, if there is jitter in the internal clock of a high speed memory device such as DDR-SDRAM, the jitter component is inevitably included in both the output signal of the device and the data strobe which is a clock based on the internal clock and used for the transfer of the output signal to the test apparatus.
However, since the conventional test apparatus judges the quality of the electronic device by one measurement, it is difficult to judge accurately due to the jitter component in both the output signal and the data strobe. In addition, if the conventional test apparatus samples the output signal outputted by the electronic device at different timing, it is necessary to store the phase data for a plurality of sampling timing signals to be produced in the test apparatus to shift the phases of the sampling timing signals by very small time intervals. Accompanying the recent trend towards a high speed semiconductor device, the search resolution of the sampling timing requires high resolution. Since the conventional test apparatus stores the phase data for a plurality of sampling timing signals to be produced in the test apparatus, it is necessary to store enormous amounts of phase data in the test apparatus to achieve high resolution. However, since it is impractical that a memory for storing such enormous amounts of phase data is provided in the test apparatus and besides storing all of the phase data of the sampling timing signals to be produced is nearly impossible, so it is difficult to test the electronic device highly accurately. Accordingly, it is desirable that a plurality of sampling timing signals whose phases are shifted by very small time intervals should be easily produced.